Selector operating circuit for use in automatic telephone systems



June 17, 1947. BAKER 2,422,283

SELECTOR OPERATING CIRCUIT FOR USE IN AUTOMATIC TELEPHONE SYSTEMS FiledMarch 6, 1945 5 SheetsSheet 1 INVENTOR GE ORGE THOMAS BAKER avg Z1;

ATTORNEY G. T. BAKER June 17, 1947.

SELECTOR OPERATING CIRCUIT FOR USE IN AUTOMATIC TELEPHONE SYSTEMS 5Sheets-Sheet 2 Filed March 6, 1943 v mveu'ron asoaee moms BAKER ATTORNEYJune 17, 1947. 5 T BAKER 2,422,285

SELECTOR OPERATING CIRCUIT FOR USE IN AUTOMATIC TELEPHONE SYSTEMS FiledMarch a, 1943 5 sheetssheet s .9 227'2 RWI B J YF H mvsmon GEORGE THOMASBAKER ATTORNEY SELECTOR OPERATING CIRCUIT FOR USE IN AUTOMATIC TELEPHONESYSTEMS Filed March 6, 1943 5 Sheets-Sheet 4 warren GEORGE THOMAS BAKERATTORNEY June 17, 1947. v GT, BAKER 2,422,285

SELECTdR OPERATING CIRCUIT FOR USE IN AUTOMATIC TELEPHONE SYSTEMS FiledMarch 6, 1943 5 Sheets- Sheet 5 Tall 1064i 0076'0/66 i fl l) i 71 A I Ig v V I I. m M I I 15 I I'fl "UH 'I p .v I p 092 k a 7 1b; 091 I I 0G I:I I I L 2 g fbb H 3.0 H66 INVENTOR esones moms BAKER Fi .5. BY 2 J AlronugY- Patented June 17, 1947 SELECTOR OPERATING CIRCUIT FOR USE INAUTOMATIC TELEPHONE SYSTEMS George Thomas Baker, Liverpool, England, as-

signor, by mesne assignments, to Automatic Electric Laboratories, Inc.,Chicago, 111., a cor:-

poration of Delaware Application March 6, 1943, Serial No. 478,243 InGreat Britain April 20, 1942 The present invention concerns improvementsin or relating to selector circuits for use in automatic telephone orlike systems and while the invention is described and has particularapplication to the selector switch described in my co-pendingapplications Ser. Nos. 478,242 and 478,245, both filed March 6, 1943, italso has application to other constructions.

The objects of the invention are concerned principally with thesimplification of individual switch circuits, the elimination of relayson the individual switches and the rendering of the switch circuits sothat they can be adapted for use with different kinds of selectors suchas group selectors, final selectors or P. B. X final selectors in aready manner. To this end the invention is directed to providing a novelexchange layout to permit of switch mechanisms fleeing interchangeable,to a novel control relay set and to novel selector switch circuits. Theinvention is also directed to the reduction of the number of relaycontacts in the switch train. In this specification the expressioncontrol relay set means a group of relays which precede a train ofswitches and exert controls over a plurality of circuits to brin aboutsuccessive operations in the switches taken into use whereby the numberof controlling relays individual to the switches may be reduced.

According to one feature of the invention a telephone system includes acontrol relay set comprising an impulse-responding relay, a hold relayand a changeover relay, each of which exerts a separate forward control,a selector having an individual control circuit such that onco-operation with the forward controls exerted by the control relay setthe selector functions as a group selector and a selector having anindividual control circuit such that on co-operation with the forwardcontrols exerted by the control relay set the selector functions as afinal selector.

According to a further feature of the invention a control relay set isprovided for successively bringing about the various steps involved inthe operation of a plurality of different kinds of selector switches(group selectors, final selectors, P. B. X final selectors) which isarranged in response to a series of impulses to exert a series ofcontrols over difierent circuits which are capable of operating aplurality of different kinds of selector which may then be connectedup,'a specific operation for each kind of selector switch being solelydependent upon the individual circuits of the selector switch which isbeing operated.

30 Claims. (Cl. 179-18) 2 Accordin to a further feature of the inventiona selector switch is employed having pairs of co-operating contacts oneof which is made when a line is selected through which a connection isto be extended, the incoming conversa According to a further feature ofthe inventionthe magnets of the switch mechanism of a selector inaddition to controlling the mechanicaloperation of the switch arearranged to effect auxiliary relay operations wherebycomplete operationof the switch andthe disconnection-of,

the magnets from the conversational leads is effected without theemployment of any relay or any like independently operated device.

According to a further feature of the'invention the magnet of a selectorswitch which is first operated is maintained operated after respondingto the first series of impulses for'the first motion of the selector,while a second magnet is subsequently opera-ted to control the secondmotion of the selector, a third magnet which eventually controls therelease of the selector being'employed for completing a circuit to thesecond magnet. I a

According to a further feature ofpthe invention a selector circuit isprovided'in which one magnet is normally connected to an incomingileadandsuhsequently during the operation of ,aswitch and on release othermagnets are connected to the same or other incoming'leads over whichleads the operation of the switch is efiected without the intermediaryof any relays. v

According to a further feature of the inven= tion a telephoneinstallation comprises a control relay set adapted to control thesuccessive operation of switches in a train, switch mechanisms withcontrol circuits of the same standard design, contact banks,cablinginterconnecting theindividual to the contact banks and havingdis-g criminating means associated therewith such;

that when one of said switch mechanisms is jacked into a contactv bankof. any stage, .the

control relay set in conjunction with the standard circuits of theswitch mechanism is adapted to cause the switch mechanism to function inthe manner determined by the discriminating means whereby the sameswitch mechanism and its individual circuits may serve for a pluralityof different kinds of switches (group selectors, final selectors, P. B.X'final selectors).

According to a further feature of the invention a selector switchcomprises two readily separable parts, the contact bank and thecontrolling mechanism, each having circuit wiring to and includingcontact-making devices'an'd the jack-in arrangement by which the circuitwiring of the two parts is joined when the controlling mechanism iscorrectly connected up to the contact bank,

cuits described relate is described in full in my co-pendingapplications Nos. 478,242 and 478,245.. The important features of thisconstruction to bear in mind are that there are no rubbing wipers otherthan what may be required for testing. The operating magnets simplyserve to release power to the switch mechanism through a cam-shapedcoupling and may therefore closely resemble telecorresponding strips inthe row constituting first According to another feature of the inventionA a selector circuit for switches having pairs of co operating contactsin their contact banks is so a :Set of co-operating contacts havingconnections onthe incoming sidel adapted for use with the'particulargroupof outgoing lines selected. Bythis'ineans it is possible to arrangea selector circuit which includes a'control relay set by which theswitchis operated and which includes the control relay set for callswithin the exchange but excludes itfor calls outside the exchange.

The invention will' be better understood from the followin'g descriptionof one method of carrying'it into effect, reference being had to theaccompanying drawings, of which correspond to similarfigures in my copending application No. 473,245. V

In'the drawings, Fig. 1 shows the operative circuits of a 19 level 10outlet per level (10/10) earth testing group selector, while Fig. 2-shows at the" left the basic control circuit therefor and at theright'the bank wiring of a'group selector.

Fig. 3 shows circuits of a 101/ 10 battery testing group selector.

Fig. 3-A shows the bank wiring or a group selector. f r

Fig. 4 shows circuits of a 10/10 regular final selector and Fig. 5'showscircuits of a 2-10 line P. B. X Ill/l0 final selector.

Fig. 6 shows circuits of the control relay set for a'train of group'selector and fiinal selector switches.

Fig. '7 'shows' circuits of the switching arrangemerits for outgoingselector levels.

Fig. 8"'shows a typical switch train embodying the circuits shown inFigs. 3 to inclusive.

In the circuits the number of contact units carried by each relay isshown underneath its designation, as for inst'ance (Fig.2) aredesignated individually in small letters whi'ch'in this instance will bebl and b2.

, A slow tof-releas e relay is shown with a hatched portion at loneendof the rectangle indicative thereof, fwhile; a slow-to-operate relay isshown with a cross at one'iend, such a relay being in someinstancesslow-to-release as well as slowto-operjat'ef The constructionofswitch to which the cir-" contacts. Of the four comb plates providedfor each set of four rows two correspond to the incoming conversationalleads, a third to the incoming private or test lead and the fourth to anauxiliary lead. Usually corresponding comb platesof each group of rowsare connected together but this is not essential and in factadvantageous use can be made of isolating one or more sets of" combplates as will be appreciated from the example given in the descriptionof'Fig. 7. The wiper or contact-making arms which effect the operationof a selected movable contact are arranged to be first moved across theends of rows automatically or under impulse control and then to be movedin either automatically or 7 under impulse control or partlyimpulse-control and partly automatically according to the function ofthe switch. For testing purposes rubbing wipers are connected to one ormore of the wiper the release magnet is released,'the wiper arms falland cause the selected teeth of the set 'offour comb plates to makecontact with the corresponding'multiple strips. The re-energisation ofthe release magnet at the end of the call is arranged to result in therelease of the switch One of the major distinctions of this selectorswitch is that the connecting function is .now

part of the bank and virtually divorced from the selector. It seemshighly desirable to maintain this condition rigidly not only forselector Sim-- plification but also fortalking efficiency. Hence allswitching contacts must be kept out of the through wires which in everyselector must look like the right-hand portion of Fig. 2 or like Fig.3A. It will be understood that the four vertically aligned contactsshown in these figures are bank contacts closed by the associated switchmecha-' nism when same has been stepped to a particular position. Inother positions to which the switch may be stepped, other similar setsofbank contacts (not shown) would be closed. Each set of bank contactswhen thus closed connects the four incoming conductors shown on the leftto a particular set of outgoing conductors like the four shown on theright. I

Referring to the bank contactsin Fig. 2, the operating circuits for theswitch mechanism which controls these contacts, among others, are shownin Fig. 1. The only controls to reach this switch mechanism comeoverrthe four incoming conductors, the operating circuits for the switch2 being connected to said conductors (and to battery and earthpotentials, as well) by means of 7 a six point jack. It will be seenthat the speaking path passes through neither relay contacts nor jack-inplug and socket points.

In designing circuits to meet this condition the following guidingfactors were taken into consideration:

1. It is extremely advisable that the holding and battery feeding bridgeshall be in the first stage or at least in an early position in thetrain.

2. The avoidance of line contacts means that selectors provided withtwo-0r more sets of wiper arms and banks should not be provided as theseneed switching relays so that generally there will be'only one availableoutlet per step on any level such as on a 100 outlet (ten levels of tenoutlets each) selector and it is proposed to use such a 100 outletselector in the new system. The new selector drive arrangements may,however, enable a secondary movement search speed to be obtained, whichwill permit of fifteen or more outlets to be hunted over in a directline. The use of such selectors also permits four wires to be used andthis fourth wire is made use of to simplify subsequent circuit design.It also simplifies the circuit problems which arise when a feedingbridge is employed at any early stage in the switch train.

3. The use of the fourth wire opens up the possibility of avoiding theuse of independent relays in the selectors. The primary and secondarymagnets may be used as relays, since they can conveniently be of thesame general design as the standard British post ofiice type relays.

4. The necessity of eliminating an extra mounting means for relaysenables the selector. to assume whatever shape is most suitable for itsown purpose and it is found that a flat type of construction with aheight only of the order of 1 inches is quite possible.

Every selector normally contains three basic relays usually designatedA, B and C. The present circuits are such that these relays need beprovided only once per train thus further justifying the use of 100outlet selectors and effecting material economies in relays.

One of the problems created by employing the battery feeding relays atan early stage in the connection is that of impulsing. In this case therelays usually subject to impulse failure are concentrated in the firststage so no difference is encountered there. The magnets are subject toan initial extra repetition due to this cause, but in any case, this ismet by the elimination of a repetition in the selectors themselves wherethe incoming impulses are delivered direct into the magnets instead ofbeing repeated from an A relay. In addition to this, the magnets areselfcorrecting and will perform a single operation on one pulse howeverlong or short.

Certain of the principles have been already laid down, and to theseanother will be added. The group and final selectors are to be kept assimple and standard as possible, and all variations are to be made inthe battery feed relay set which is jacked in as a separate unit andserves to control successive switches. Apart from the question ofstandardisation one of the main advantages of this type of layout is thefact that any new technique can be introduced without affecting themechanical parts of the exchange. One example of this as is describedlater is shown in the introduction of the sped timing type of impulsingcircuit (United States Patent No. 2,334,591, granted November 16, 1943,to Taylor and Baker) by means of which impulse-responding circuits areadapted to function independently of the ratio of received impulses, theseparate time-measuring functions of the ordinary B and C relays beingcombined into one timing function which simply measures the overalllength of each impulse instead of the length of the separate partsthereof as hitherto. With the normal type of exchange layout it wouldbenecessary to modify every selector in the exchange to introduce thisfeature. Now it can be incorporated or omitted merely by changing thefirst relay set. With the increased lengths of connection over whichdialling is being effected and the more general use of A. 0. methods,this point is very important.

Detailed circuit consideration of Figs. 1 and 2 will now be given and itwill be understood that th basic control circuit shown on the left inFig. 2 is permanently associated with the contact bank of the firstgroup selector shown in Fig. 1; it controls not only this selector,however, but also all switches at subsequent stages in the switch train.Briefly, the combined primary magnet and relay VR (Fig. 1) is impulsedfrom the control relay set over the negative line, only one step beingtaken by the switch for each up and down movement of said primarymagnet. The Z wire is used for primary/secondary changeover, which isbrought about by the combined release magnet and relay ZR. Uponinitiation of the secondary movement relay VB. is maintained from theprivate conductor P and serves as a switch holding relay; on its releaseat the end of the call it reenergizes release magnet ZR to initiaterelease of the switch. Since the switch is to function as a groupselector, i. e. with dialled primary movement and self-driven secondarymovement, and is to be controlled from a simple control circuit such asthat of Fig. 2, then the primary driving cam will have to be of thetwo-stage escapement release type while the secondary driving cam willhave to be of the single stage simple release type.

When a subscriber calls the control relay set or group of Fig. 2 isassociated with the calling line via any suitable apparatus which mightconveniently be of the line finder type. Relay A therein operates,followed by relay B, whereupon the selector switch permanentlyassociated with the control relay set is prepared for operation by theextension of earth forward over the negative and P leads. The earth onthe negative lead operates the primary control relay VR, correspondingto relay M in the aforementioned application Ser. No. 478,242, which atcontacts ml prepares a locking circuit for itself on to the private P.The armature extension on relay VR allows the associated driving cam torotate slightly to the second stop position and when relay A drops atthe beginning of the first impulse relay VR releases in turn and freesthe driving cam. Under local spring pressure it rotates further andengages with the roller and thereupon completes a revolutionindependently of the control relay VR as already described. The releaseof relay A at the beginning of the first impulse also causes contact :12to energize relay C. On the next and subsequentimpulses of the train theprimary carriage is operated in a similar manner to cause it to advancea number of steps corresponding to the digit dialled and it will benoted from Fig. 2 that relays B and C are held operated throughout thetrain by virtue of their slugs, the B relay guarding the connection andmaintaining the P lead earthed while the C relay maintains the Z leadopen to prevent the premature operation of the release relay ZR which isconnected up to the Z lead on the first primary step due to themechanical operation of the switch off normal contacts NI. 7

At the end of the first impulse train relay C releases after its slowrelease period and energises relay ZR which lifts the wipers clear ofthe bank level in question and at contact 272 provides an operatingcircuit for the secondary control relay RR. This relay in operatinglocks relay ZR at contacts rrl and also releases the catch on thesecondary driving cam which engages with the roller. Thereupon, since inthis case the catch arrangement. between the extension of the secondaryrelay armature and the driving cam is not of the escapement type, thenuntil such time as the motion of the cam can be halted by the release ofthe relay RR. and the consequent interposing of the tip of the extensionarmature into the path of the stop on the driving cam, the cam willcontinue to be rotated by both the driving roller and also periodicallyunder control of its local drive-engaging spring. This will cause thesecondary carriage to advance step-by-step and carry with it the primarycarriage; thus the wiper arms secured thereto are caused to enterstepby-step into the selected bank level. On the first secondary stepthe secondary off-normal contacts NR! and NR2 are mechanically operatedso that the holding circuit of relay VB is transferred to the P lead atcontact NR2 and is locked for the remainder of the call via resistanceVRR.

At the end of each secondary step the operating circuit of relay RR isbroken by the interrupter contacts rl, which it will be understood aremade during the engagement of the secondary driving pawl with thesecondary ratchet, but when this occurs it will be noticed that thesecond winding of this relay will be connected over a rubbing test wiperP on to the corresponding lead of an outgoing trunk. If such a lead isbusy an earth po- I tential will be encountered thereon so that relay RRwill remain held and the secondary carriage will step on to the next set*of outlets.

When a free set of outlets isencountered the P wiper will not encounterany earth potential and the interrupter contacts rl on opening willcause relay RR rapidly to deenergise and the catch on its armature willthereupon fall into pothrough to the succeeding selecting stage, whichmay be either the final selector stage or another group selector stage-If the latter the relays A,

B and C will operate the subsequent group selecto normal under localspring tension. The mechanically operated springs restore to normalandat contact NRI relay ZR is released to render the selector ready forfurther use. a

If all outlets on a level should prove to be busy the wipers willadvance to the 11th step position in which position the 11th stepcontacts SI will be mechanically operated and will open the circuit ofrelay RR to halt the secondary motion in the same manner as for a freeoutlet so that the switch will contact the control relay set through tothe 11th set of bank contacts. It will be understood that the 11th steppositive contact is permanently earthed so that earth will be extendedback over the positive lead to the control group which in responsethereto will initiate the return of a busy tone to the calling party ina manner discussed in the previously mentioned application. If anoverflow for the level is to be provided this could be connected to the11th step P contacts and the contacts SI in operating could v be in theform of a changeover combination tor in the same manner as previouslydescribed;

If th former the operation of the battery feed relay set will be asdescribed except that it will function twice while the circuits will beappropriately modified from those shown in Fig. 1. More detailedcircuits are described later with reference to Figs. 3 to 6.

If the calling party should now hang up, relays A and B in the controlrelay set will release and the removal of earth from the P lead willrelease relay VR, whereupon an operating circuit for relay ZR isre-completed via contacts m2 and NHL bank and as previously described,the primary and secondary carriages are now freed to restore The wipersare again lifted clear of the a adapted to extend earth forward over therub bing P wiper to operate the overflow meter in this condition.

Consideration will now be given to the battery testing types of selectorswitches in which the idle condition is indicated not by the absence ofearth but by the presence of negative battery. Such switches normallyinvolve more relays than, the earth testing variety, since the fast cutdrive relay could not operate quickly enough if it carried the switchingload and a relief thereon must therefore be usually provided. In thepresent instance however, no more relays than usual are required, sincethe cut drive relay can be located in the control relay set, while theswitch banks themselves provide the switching function.

The circuits of a ten level ten outlet per level (16/10)- batterytesting group selector (Fig. 3), a 10/10 regular final selector (Fig. 4)and of a 10/10 P. B. X final selector (Fig. 5) with P. B. X groups of2-10 lines will be considered in turn in conjunction with the controlrelay set circuit of Fig. 6.

A typical switch train including these units in their properrelationship is shown in Fig. 8. It will be observed that the finalselector in the switch train may be astandard one employing the circuitsshown in Fig. 4' or may be a P. B. X

final selector employing the circuits shown in Fig. 5. As a matter offact, the trunks outgoing from certain levels of the group selector canex- 7 tend to final selectors of one type while the trunks outgoing fromthe remaining levels of the group selector can extend to final selectorsof the other type. It will be obvious thatin large systems the switchtrain may include, between V the first group selector and the finalselector, one f or more additional group selectors identical to the oneshown. On the other hand, in small systems the group selector may beomitted, the.

could also be applied to the primary movement I were it not for the factthat the primary control relay also serves to hold the switch by beingmaintained from the P lead after initiation of the circuit of Fig. 2 isreplaced by the speed timing impulse respo'nding circuit shown anddescribed in the before-mentioned Patent No. 2,334,591, this having beendone in order to avoid the use of the normal timed B and C relays andalso because it may be said that the Ipresent standard ir'n-pulsingmethods have about reached the limit of their usefulness as evidenced bythe increasing use which is being made at present of electro 'mechanicalimpulse regenerators in telephone exchange systems. In speed timingarrangements the impulse speed is measured instead of impulse ratio ashitherto, so that line conditions, which have a distorting effect onimpulse ratio but not on their speed, are now an almost negligibleconsideration.

The speed timing circuit comprises a single time measuring circuitincluding a condenser QD, resistance Y and neon tube NT and is adaptedto compare the total length of the makeeplusbreak period of each impulseof a received train with a predetermined period of the order of 150milliseconds which is substantially equal to total length of eachimpulse at the lowest impulsing speed tolerated in practice.

The circuit is set into operation at the beginning of the first-impulseof a train and is restored to normal by each successive impulse of thetrain until the last pulse is received, after which it functions eitherto initiate release of the connection or to render the circuit of theswitch in question ready for the further function required'of itaccording as to whether the impulse train finished with the impulseaccepting relay normal or operated.

When a calling subscriber is connected through to the control group,relay AA operates, and at contacts aai brings up relay BB, whilecontacts aa2 rapidly charge up condenser QE to the exchange batteryvoltage via resistance YM. Relay BB in operating at contacts b'b'lplaces aiguarding and holding earth on the incoming P lead and atcontacts bb l extends earth potential out over the outgoing (right hand)negative lead to operate the primary relay IVR in the first "groupselector associated with the control :group, the circuit for which isshown Fig. 3.

Referring now to Fig. 3 relay lVRin operating at contact 1121*!disconnects the idle resistance marking battery comprising the batteryconnected resistance YA which is normally connected to the P lead toindicate the free condition of the switch to a preceding hunting switchwhere such a switch is involved. The armature extension of relay 'IVRallows the associated driving cam to rotate slightly to the "second stopposition and the switch is now 'readyto receive a train of impulses.

Meanwhile relay BB in operating at 19192 connccts up dialtone-over'common lead DTE to the centre winding "of relay AA andthe'dialtoneextends by induction on to the incoming speaking leads andthence to the calling party.

When relay AA releases on the first impulse-of a dialled train relay Eenergises in series with relay BB which is therebyheld. RelayE inoperating at cont-act e l brings up relay M'D which atcontacts md3disconnects the dial tone circuit and locks via contactsmd-ldt and bet:to earth.

Contacts aa2 allow the charged condenser QE negative wire to interruptthis circuit fora period long enough to release, the selector relay .IVRand so to free the driving cam and cause the selector to proceed toperform one primary step. This rather unusual method of impulserepetition which is more correctly regeneration in another form isprimarily adapted to operate in conjunction with the secondary movementsimple catch release control so that this movement may be controlled bydialled impulses or may operate independently in a self-driven circuitaccording to the function required of it. Details of such operationswill be :given later.

When relay AA re-operates at the end of the first impulse relay BBremains held Via relay E in parallel with resistance'YL, relay E-alsoremaining operated under this condition and at contacts aaz condenser'QE is rapidly rechargedvia resistance YM. The second break impulse ofthe train therefore re-operates relay HR for a short period in the samemanner as for the first, andthe second momentary b-reakis made in theearth circuit over the negative lead to the selector relay 'IVR,whereupon a s'econds'tep is delivered to the selector, This happens foreach succeeding impulse of the train until at the end of the train relayremains operated for the comparatively long-1 nterdigital pause period.

During this period the initiation of the selector secondary movement isperformed by the impulse speed timing circuit in the following manner.(3n

the first operation of relayE, i. e. at c'ommencement or the firstreceived break impulse, a charging circuit for the speed timingcondenser 'QD is prepared at e3 via resistance YO to positive loatteryand when reIayJHR releases after extending the momentary break impulseto the selector condenser QD starts to charge. The respective values ofthe resistance and condenser are such that the condenser takesapproximately milliseconds to reach a voltage large enough to flash theneon tube NT. More precisely, the charging period is 150 millisecondsless the operated time of relay HR which operates for fa, given time:period at commencement of the break period of each :recei'ved impulse.

If the second break period is received before the elapse of 150milliseconds from the commencement of the first break period then relayHR in again'operating at contacts hr! will rapidly discharge condenser'QD via resistance YM and the charging up of this condenser =willre-commence on the releaseof relay-HR. The discharging and charging ofcondenser QD'occursifor ea-ch re ceivedimpulseuntil at theend-of theitrain relay AA remains operated and allows the voltage of condenser QDto reach the flash voltage of neon tube NT 150 milliseconds afterthe-commencement of the last break impulse of the train.

Neon tube NT in flashing brings :up relay CC and this relay in operatingJocks over its upper winding in series with resistance and :62 and atits contactsccl disconnects 're'lay E which-commences to release slowly.:At contacts ce-2 battery via the winding of relay D is ifed out alongthe Z wire and since the selector primary off-normal contact 4N2 willhave closed when the switch stepped off normal in "a primary directionthis battery potential extends through to the combined release -mag-netand relay IZR. Relays D and lzR operate under this condition,

' [Relay lZR in'operating lifts the switch wipers clear of the level ofcontacts into which they are to be inserted and at contacts lzrloperates the secondary control relay IRR from the earthed negative leadto initiate the self-driven secondary driving movement of the switch insearch of a free outlet to a succeeding switch.

Relay lRR in operating initiates the secondary drive by freeing'thecatch on the secondary driving cam and locks relay ZR via contacts lrrlto battery Via resistance 'YC. On the first secondary step the secondaryoff normal contacts lNRl and INR2 are operated, the former disconnectingthe initial operating circuit of relay lZR so that relay D in thecontrol relay set releases without having effected any circuit operationat this stage; at contacts INRZ the holding circuit for relay IVR istransferred to the earthed P lead.

Contacts lzrZ connect the rubbing private test Wiper RWl through overthe positive trunk and thence via resting contacts fl and M, operatedcontacts 005 and resistance YQ to the upper winding of the fastcut-drive relay HR.

.When the rubbing wiper RW! encouters an idle outlet as indicated by anidle resistance marking battery such as that normally connected to the Pwire of this switch, relay HR rapidly operates and at contacts hr?disconnects the earth potential being fed out on to the negative lead,whereupon relay IRR releases to stop the switch motion and at contactslrrl de-energises relay iZR to drop wipers into action with the banks soas to switch the control relay set through to the succeeding selectorstage. The control group relay H has not enough time to operate at thisstage, for a reason to be later described.

The secondary drive interrupter contacts lrl function in thesame manneras for the earth testing group selector case previously described.

Returning again to the control relay set it will be noted that whenrelay CC is operated to initiate the secondary movement it remainslocked either until the expiration of the slow release time of relay Ewhen its locking circuit is opened at contacts e2 or of the timerequired to find a. free trunk when its locking circuit is opened bycontacts hrl, whichever is the longer.

When the selector relay IZR is disconnected to switch the control relayset through to the succeeding selector stage contacts lzrZ open theoutlet testing circuit and so release relay HR in the control relay set,the succeeding selector taken in use being guarded by the extension ofearth thereto over the P wire from the control relay set. On release ofrelay CO the control set will be in readiness for the dialling of thenext digit to control the positioning of the succeeding selector noWtaken into use.

If the subscriber should now hang up relay AA will fall and relay E willoperate in series with relay BB whichholds. discharged through relay HRand a single impulse will therefore be sent out over the negative wireto the succeeding selector. Approximately 150. milliseconds after therelease of relay AA the speed timing circuit will come into operation,whereupon relay CC operates and since relay AA is now normal the holdingcircuit of relays E and BB is disconnected and the former will commenceto release slowly due to its slug while the latter will releasecomparatively slowly due to its non-inductive resistance shunt involvingresistor YN. On release of relay BB the locking circuit for relay CC isopened at bbl and on'release of relay E the holding earth is removedfrom the Condenser QE will be 12 P lead at e'l whereupon relay IVR whichhas been previously held is now released and at contacts lorZ completesa re-operating circuit for relay IZR, whereupon release of the groupselector is effected in the same manner as for the previously describedearth-testing group selector.

As regards the succeeding selector, on seizure its primary control relaywill have been operated over the negative lead and on the release ofrelay AA the momentary break in the earth potential extended on to thislead causes the primary control relay to release to initiate one primarystep. 150 milliseconds later relay CC operates, and during the slightlyslow to release time of relay BB battery potential via the winding ofrelay D is fed out over the Z lead to bring up the release relay and thesecondary control relay is now operated from the negative lead on towhich the earth will have by this time been replaced. The switch nowcuts in and proceeds to complete the first secondary step and if relayBB should release during this time then it will be seen that theinterrupter contacts equivalent to lrl, which are made during thestepping operation, maintain the release relay independently of the nownormal secondary control relay. At the end of the step the interruptercontacts open and release the release relay, whereupon the incomingprove busy then the secondary movement will continue until the rubbingwiper RWI reaches the 11th step position, the P contact of which ispermanently wired to a resistance battery so that the cut-drive relay HRwill operate to release the secondary control relay IRR and so halt therotarymotion. Relay E will have released by this time and hence relay HRin operating will release relay CC and contacts hrl. In the selectorrelay IRR in releasing releases relay IZR and the 11th step contacts ISIthereupon become effective to extend earth back over i the positive leadto bring up relay G in the control group. Relay G in operating looksover its contacts gl and at contacts g2 connects up a busy tone earthsource over common lead BRE to the centre winding of relay AA fromwhence it is returned to the calling party.

As regards the 11th step P contact the outgoing P contact of which ispermanently wired to resistance battery, it will be understood that theincoming P contact of the bank switching pair will have been removed sothat when the wiper arm is dropped on the release of relay I ZR theoutgoing contact will not be earthed from the control relay set andfurther selectors can therefore switch to the same battery. On therelease of relay IZR the 11th step contacts ISI also become effective toextend earth to an overflow meter which will be connected to the 11thstep outgoing positive bank contacts,

It should be noted that when controlling a' group selector the negativebattery encountered when the selector relays IRR and lZR release 7control set is governing the setting of a group selector; underconditions where the control set is governing the setting of a finalselector relay H will be given ample time to operate as will be laterdescribed.

Assume now that when the group selector has been seized the subscriberhangs up without dialling. In this case an impulse train of a singleimpulse is simulated but with relay AA remaining normal at the end ofthe impulse. The release of the selector follows along precisely similarlines to that of the succeeding selector when the calling subscriberhung up after dialling one digit only.

It will now be assumed that the succeeding selector taken into use is aregular final selector, circuits of which are shown in Fig. 4. The bankswitching portion associated with the final selector is the same as inFig. 3A, and the manner in which the two are associated will be clearfrom Fig. 8.

On seizure the primary relay 2VR is operated and the initial operationof this switch up to the energization of the release relay 22R at theend of the first received train of impulses is the same as for the groupselector of Fig. 3. Relay 2ZR in operating connects the secondary relay2RR up to the earthed negative lead, but in this instance since relayZRR. is connected to earth instead of battery it will remain normal dueto the shunting action of the earth on the negative lead, which earthalso serves to hold the vertical relay ZVR. Hence no automatic secondarymovement takes place and the control group relay D remains held inseries with relay ZZR.

When the next train of impulses is received by the control group, theneach time relay HR operates for a short period at commencement of thebreak period of an impulse the earth potential to the negative lead isreplaced by a battery potential via resistance YP and the selectorsecondary relay ZRR for each received impulse, is operated for a lengthof time governed by the operated time of relay HR.

It will be remembered that the secondary driving cam is of the simplecatch release type and hence in order that dialled impulses may beeffective thereon it is necessary to ensure that the control relay 2BRafter having been operated in response to an impulse shall release intime to put a stop in the path of the secondary cam before this cam hascompleted the one half revolution necessary to advance the wipersthrough one step; for this purpose the length of the secondary pulse asgoverned by relay HR is ar-' ranged to be less than the 30 millisecondperiod required by the cam to complete one half revolution. Eachsuccessive pulse is similar to the first and the carriage is thusdialled step by step into the bank.

In the control group at the end of the first impulse train relay CO inoperating initiates the slow release of relay E in usual manner, whilerelay BB remains held. One locking circuit of 14 relay CC is opened byoperated contacts dl so that when relay E releases after its slowrelease period relay CC will immediately restore and the circuit is nowin readiness for the next dialled digit.

When relay CC again operates at the end of this next digit the earthedtesting relay HR and the switching relayHare connected up in series overthe positive wire and thence via contacts 2zr'2 and the rubbing privatetesting wiper RWI to the called subscribers P conductor.

If the called line is free a resistance battery marking will beencountered on this conductor and relay HR will operate in seriestherewith; in this case, as distinct from the group selector, since theselector relay ZZR is locked up to relay D relay HR in operating will beunable to cause the release of relay ZZR, and accordingly the switchingrelay H will get time to operate during the slow-to-release time ofrelay E which will have again commenced to release slowly on theoperation of relay CC. On all final selector switches (ordinary and P.B. X) three leads extend to the line circuit of each subscriberconcerned, the negative and positive outgoing bank contacts connectingwith the corresponding negative and positive leads of the line circuit,while as regards the subscribers private leads, these connect up withthe outgoing Z bank contacts, the outgoing 1? bank contacts not beingwired out. Hence, when relay H operates contacts h5 extend earth forwardover the Z wire to short circuit and release the final selector relayZZR and so efiect the switching through of the control set tothe calledsubscribers line. The earth will also extend forward over the switchbanks to the called subscribers private wire to mark this line as busyand so effect the necessary control in the subscribers line equipment toreceive the income ing call. The connection to the called subscribersprivate wire is initially over wiper RWI and subsequently when ZZR isreleased over the main contacts.

Contacts MI and h! respectively, connect up an earthed ringing generatorGNR via the winding of relay F on to the negative line and a ringingreturn battery via lead RRB and resistance YS on to the positive line soas to ring the called partys bell, while contacts hZ connect up aringing tone earth source over lead RTE to the centre winding of relayAA from whence the ringing tone returns to the calling party.

When the called subscriber replies relay F operates and at contacts flremoves the short circuit from its right hand winding over which itlooks, while at contacts fB'and f! the ringing is tripped and thebattery feeding relay D is connected at f4 and f5 across the outgoinglines and through to the called party. Conversation may now proceed.

It will be remembered that relay MD was operated from relay E when thelatter operated in response to the first train of impulses received bythe control group and when relay D operates it will be seen that theholding circuit for relay MD is opened at contacts d3. On release ofrelay MD after its slow release period a battery potential viaresistance YK is extended backwardly over the metering lead M toregister the call against the calling party.

At the end of conversation when the calling party hangs up relay AAreleases, whereupon relay E operates in series with relay BB whichholds. The speed timing circuit comes into operation and millisecondsafter release of relay "AA relay CC" comes in and initiates the slowrelease of relays E and BB. On release of relay BB relays H, F and D arereleased, while on release of relay E the holding earth is removed fromthe P lead and the various primary control relays in the selector trainwhich have been held therefrom are released so as to bring up theirrelease relays and so initiate release of their respective switches.

Assuming now that the required line proves to be busy then relay H willfail to operate during the release time of relay E and when relay Ereleases relay CC will fall and will connect up relay G on to thepositive lead. Relay G thereupon operates to the earthed busysubscribers privateencountered by rubbing wiper RWI and returns busytone to the calling party at contacts 92, while at contacts g4 thecircuit for extendin battery potential via the winding of relay D on tothe 2 lead is opened and --the final selector release relay 22Rthereupon releases. The final selector wiper arms are thereupon loweredinto contact with the bank but it will be appreciated that interferencewith the busyrequired subscriber is guarded against by thedisconnection in the control group of the negative, positive, and Zwires at contacts g5, and g4 respectively, which wires are, in the caseof all final selector switches, respectively connected up at the finalselector switch banks to the subscribers negative, positive and P leads.The P lead from the control group and through the selector train to thefinal selector remains earthed for the purpose of holding the selectortrain until the calling party hangs up, and since the P lead does notextend through to the subscribers line circuit it can do so withoutintereference to the line of required subscriber.

It will now be assumed that the last two dialled digits are effective onthe 2-10 line P. B. X final selector shown in Fig. 5. The primary andsecondary setting of this switch is the same as for the regular finalselector but in this case,

when at the end of the last impulse train the V secondary control relay3BR is released from the control group, relays HRand H are connected inseries via the positive trunk, operated contacts 3.212, resting contacts3TH, the rubbing test, wiper RWI and the contact it is resting on to theprivate wire of the line circuit in the wanted P. B. X group to whichthe final selector wipers have been positioned] The contacts engaged byRWI are connected to the outgoing Z contacts which as in the case of theregular final selector are connected through to the subscribers privatewire. It will be noted in addition that the Z wire is connected throughto a corresponding contact on an auxiliary bank via the oil normalcontacts 3N2, condenser QA, contacts 321?, contacts 377'2, and therubbing wiper RWZ. The auxiliary bank which is only used for testingpurposes may comprise the outgoing P bank contacts which on the finalselector are normally unwired while the outgoing Z contacts connectthrough to the subscribers privates. On this auxiliary bank it will beunderstood that the first lines of P. B. X groups are marked by a sourceof battery with alternating current imposed, on intermediate lines theoutgoing P and Z contacts are strapped, while on the last lines aresistance battery is connected.

If the switch wipers are positioned to the first line of a P. B. X groupthe battery connected alternating current source encountered by wiperRWZ extends back via condenser QA and the Z lead to operate relay HS onits left hand winding 16 in series with resistance YP, this winding ofrelay HS being arranged in association with condenser QF and rectifierMRA so as to operate satisfactorily on alternating current. Relay HS 'inoperating at contacts hsZ maintains a holding circuit for relay E sothat relay CC in-turn will remain locked after operation from the speedtiming circuit, while at contacts hsl it prepares for the extension ofbattery potential via its other winding on to the negative lead.

If the line is found to be free by test wiper RWl the free line batterypotential will extend back over the positive lead to bring up relay HR.In this case the operation of relay HS will be without effect as thecircuit to the negative lead is opened at contact hr3. Relay H inoperating after the operation of relay HR will, in a manner as alreadydescribed in connection with the regular final selector, cause the P. B.X final selector to switch through to the first line on the P. B. Xgroup after which ringing is carried out in the manner alreadydescribed.

If the'line is busy however, relay HR will fail to operate and theresistancebattery extended on to the negative line via the right handwinding of relay HS will bring up the selector secondary relay 3BR tocause the switch to hunt automatically over the lines in the group.

With relay 3BR. operated it will be noted that at contacts 3TT| thepositive wire'is transferred from the testing wiper RW! to wiper RWZ,but on I movement the control group will still be virtually testing overthe subscribers privates of the group. Itwill be appreciated that thealternating current circuit over wiper RW2 can operate independently ofthe D. C. holding circuit for relay 32R since it is isolated therefromat the selector by condenser QA and at the control group by condenserQF. When a free intermediate line in the group is encountered relay HRenergises and releases the selector secondary control relay 3BR to stopthe movement. Relay H then operates and functions in the manner alreadydescribed to cause the lines to be switched through.

If the first and intermediate lines of the dialled P. B. X group areengaged the selector on reaching the last line will atfits Wiper RW2encounter a resistance battery potential which marks the last line ofall P. B. X groups. Accordingly, relay HR willoperate over the positivelead and over operated contacts 321'2 and 3TH whatever the condition ofthe corresponding line (the relevant outgoing P and Z contacts not ofcourse being strapped on the last line of a P. .B. X group).

Relay HR in operating at contact h13 opens the the last subscriber inthe group so that if the line is free relay H will operate to effectconnection with the called line. If the line is busy relayH will fail tooperate but relay HR. will remain held over its lower winding to batteryvia resistance YM. On release of relay El after its slow release periodrelay CC will fail and will release relay HR, while'at contacts cs5relay G will be 'conn m A. 11*; .iJi 4i nected up. to the positive leadand will then operatetoi the busy earth potential encountered, by thebusy test wiper RWl' with results as already described.

Incase a particular line outof a P. B. X group is required, as undernight service conditions, then. provided that this is not thefirst lineof the group, if the P.v B. Xselector is positionedaccordingly'bydiallingno P. B. X huntin will take place ifilllSTfOllIld' to be busy since relay HS will not have been operated toprovide the, conditions necessary for P. B. X hunting and the operationsinvolved will be as for a regular final selector when an engaged outletis encountered.

It is to be noted that the circuit of Fig. can be used for a switch tofunction as Well as either a group selector, final selector or P. B. Xfinal selector by simply'making appropriate connection to the contactsengaged by the rubbing Wipers and in the case of a group selectorarranging that the rubbing wipers RWZ engage a normal contact connectedto alternating current supplied in series with battery as in the case ofthe first contact of a group.

Other points and additional features both in regard to the selectorswitches and the control group will now be discussed.

As. previously mentioned theoutgoing'Z bank contacts in the finalselectors connect with the P conductors of the subscribers line circuitsconcerned so that when the control group switches through earth will beextended forward over the Z wire from contacts M to the P conductor ofthe subscriber in question. The Z wire is carried through between thecontrol group and the final selector without shunt or seriesresistances, so that various facilities such as toll breakdown whichwere originally provided from. the final selector can nowbe provided inthe control group.

If the circuits of the regular final selector and group selector shownin Figs. 3 and 4 are examined and compared, it will be seen that thereis no reason why the final selector circuit should not be employed onthe group selector provided that in the control group the Z circuit isopened to release magnet ZZR. by relay HR when it opens the circuit formagnet ZRR On an idle line being found. Certain other modifications willbe required in the control group to ensure that relay HR does not openthe circuit of ZZR other than at the end of an impulse series or at theend of hunting. With such circuit modifications the necessity ofmaintaining the circuit of ZZR by 2ZR3- is avoided and the cir cult ofFig. 4 may be simplified by connecting ZZR direct to 2N2. It is obviousthat it is impracticable to illustrate all the modifications which arepossible and it is only proposed to indictate generally the lines onwhich modifications may be made as a circuit designer skilled in the artcan readily make the necessary complementary modifications. For instancewhen the primary movement has been concluded relay- ZZR will be operatedas previously, and the control group will be arranged to extend aresistance battery out over the negative lead to operate relay 2RR viacontacts of which the primary relay 2VR will be maintained temporarilyuntil the secondary drive commences and the operation of relay 2BR. willcause the secondary drive-to take place. When a free line is reached thecutdrive relay HR will operate through the positive lead as before andwill thereupon open the negative lead to drop relay ZRR and in thisinstance the Z wire will be also opened by contacts of re- 18 lay RR to.release relay ZZR and so effect switching through to'the succeedingswitch. In the case. where the. control group will be working intoafinal selector it, will operate almost exactly as shown and described.

In, the case described with regard to Fig. 5 it is possible to insertthe switch mechanism in any positionin an exchange and it willautomatically function in the manner required of it, i. e. as groupselector, final selector or P. B. X selector. In thecase of Fig. 4 ithas been explained how this circuit can be adapted to be used either asa group or finalselector but in this case a certain amount ofdiscrimination is necessary. This discrimination may take two forms, oneinvolving the modification of the circuit of the control groupwaccordingto discriminating signals sent back to the. control group'according tothe kind of switch taken into use so that the operation of thecontrolgroup is adapted in each case to the kind of switch being controlled theother involving a modification of potentials. applied to one or more ofthe magnet relays of the switch when it is. plugged into position, forinstance relay [RR is connected in one case to battery potentialand 2BRis connected to earth potential if the winding. of IRR or 2BR wasconnected to a pin of the jack and engaged with a terminal in thecontact bank. This terminal could be connected to earth in the case of afinal selector and battery in the case of a group selector.Discrimination inthe first case. may be eifectedby means such as aresistance inserted at the jack of the group selector andnot at the jackof the final selector or vice versa. Proceeding along: the linesindicated, one approves of the result that an identical detachableselector circuit is used for both-groupandfinal selectors in which caseevery selector in the exchange could either perform its secondary motionautomatically or under the in.- fiuence of; dialled impulses accordingto the discrimination on the jack.

A group'selector could furthermore be adaptedto operate in the samemanner as a P. B. X final selector, i. e., with an initial dialledsecondary movement, andthis facility might be used in. connection withtesting and routining functions and for special trunking arrangements.For instance, the selector can either start tohunt from the first bankcontacts or can be dialled to a position on the bank and be caused tohunt on from there.

Another point arises in connection with the selecting. switchesaccording to the invention, andthat is that they can be renderedself-routining, since each selector can derive from the drivingroller-the requisite trains of impluses at the raterof ten per second.An unlocking push button at the front of the selector could apply theimpulses to' the primary control relay in response to which? the switchwould; step in a primary directiongto say level 9, where mechanicallyoperatedsprings would energise the release relay to cause the switch toperform a secondary hunting movement until contact 9 on this level isreached. Further springs would then be operated to halttheswitchmovement.

It will benoted that no mention hasbeen made of P. B. X groups havingmore than ten lines. Such large P. B. X groups are the exception ratherthan the rule. posed to employ selecting units catering for up to100.1ines: and which by virtue of the use of one or; moresubsidiaryrelays are capableof performing hunting operations over alevel, then of being In these. cases it is pro- 19- released backagainto the commencement of the level and stepped up to the next level wherethey perform another hunting operation, and so on; a method of achievingsecondary release without primary release bein described previously.

As regards the control group, it will be noticed that the speechtransmission condensers are not introduced into the circuit untilimpulsing has been completed so that the distortion ordinarily producedtherefrom, which is especially troublesome when impulsing over longjunctions is avoided. v

A facility which has not yet been described in connection with thecontrol group is that of switching out the transmission bridge when thecall is to be routed over an outgoing junction. This'can be done in asimple manner which will involve relay contacts in the through leads butthe following method now to be described in connection with Fig. '7 inwhich relay contacts in the through speaking leads to the outgoingjunction repeater relay sets are entirely avoided, is proposed.

Referrin to this figure, it will be understood that normally on aselector bank the comb plates relevant to each of the various leads,say, negative, positive and private, are strapped together behind themultiple, but where say levels 1-9 of a first selector are to giveaccess to the local switching train, and where level is to give accessto 20.? outgoing repeaters leading to other exchanges the onlydifference bein that instead of the control relay set leading to thefirst nine levels it will only be connected to the level leading tolocal switches while the other levels will be connected direct to theincoming leads. The circuit arrangements of Fig. 7 will equally apply tothis arrangement. Similarly it is possible to arrange for more than onelevel to be used for the local switch train and more than one level tobe used for connections to other exchanges again employing the circuitarrangements of Fig. '7.

I claim:

In a switching system, an automatic switch having access to a pluralityof lines and adapted to take a step by step primary movement followed bya step by step secondary movement in order to select a particular one ofsaid lines; control equipment; an impulsing circuit extending from saidcontrol equipment to said switch; a circuitextending from said controlequipment tosaid switch for causing said. switch to change over from itsprimary movement to its secondary movement; and means including saidcircuits whereby each step of the primary movement of said switch isalways directed by said equipment, whereby each step of the secondarymovement of said switch at times also is directed by said equipment, andwhereby said secondary moveoutgoing junctions, the comb plate wiring maybe 7 split so that the incoming speakingleads will connect via contactsog3 and og4 of a relay 0G via the control group of Fig. 6 with the combplates 1-9 and will connect directly with the comb plates of level 0.

If any of the digits l-9 are dialled, the selector train will be set upin rnormal manner under control of the control group of Fig. 6 asalready described, but if digit 0 is dialled for the purpose ofanoutgoing junction call, then the directly connected level combplateswill come into operation as follows: When afree outgoing repeater-isfound, relay 0G will be operated from earthedcontacts bb of the controlgroup relay BB over its left hand low resistance winding to the idleresistancemarkin battery which will be connected to the incoming Pleadat the outgoing repeater; the low resistance winding of relay 0G servingas a guardin potential and also to operate relay 0G which thereupon atcontact 093 and og4 disconnects the circuit to the control group. Duringthe release of the control group, which will take place at approximately150 milli-seconds after the opening of the loop thereto, the subscribersloop will be extended forwardto oper ate the impulse accepting relay IAand in turn the guard relay IB in the outgoing relay set and a guardinand holding earth will be extended back to hold relay 0G on its otherwinding, be-

,fore relay'BB in the control group which has previously been holdingit, releases. The subscriber will now be directly connected through tothe outgoing repeater which will receive the remainder of the trains ofimpulses to be dialled and which will provide the battery feed to thecalling subscriber. The selector will of course remain held over the Plead from earth at the outgoing? relay setwhi'ch will extend viacontacts 0g2 and through the control group to the selector primarycontrol relay.

Theiabove arrangement can also apply to the case where the groupselector has only one level giving access to the local switches in theexchange and'th'e other levels all have access to ment at other times isa hunting movement controlled by said switch.

2. In a switching system, two automatic switches each having access to aplurality of lines and each adapted to move step by step in a primarydirection and then step by step in a secondary direction in order toselect a particular one of said lines, common control equipmentconnected to one of said switches at times over a set of conductorsoutgoing from said equipment and connected to the other one of saidswitches at other times over the same set of conductors over from itsprimary movement to its secondary movement, and means including saidcircuits whereby each step of both said movements of one of saidswitches is always directed, by said equipment, whereby each step ofboth said move-- ments of the other said switch is sometimes directed bysaid equipment and whereby one of said movements of said other switchsometimes is a self-controlled hunting movement.

3. In a switching system, a plurality of automatic switches each havingaccess to a plurality of lines and each adapted to move step by step ina primary direction and then step by step in a secondary direction inorder to select a particular one of said lines, common control equipmentconnected to the difierent ones of said switches at difierent times overthe same set of conductors outgoing from said equipment, an impulsingcircuit extending over certain of said conductors to the connectedswitch at each of said times, another circuit extending over certain ofsaid conductors to the connected switch at each of said times forcausing the connected switch to change over from its primary movement toits secondary movement, means including said circuits whereby each stepof the primary movement of the connected switch always is directed bysaid equipment and the secondary movement of the connected switch is ahunting movement controlled by that switch, and a third circuitextending over certain of said conductors at each of said times fortesting the lines encountered by said switch during said huntingmovement.

4, In a switching system, two automatic switches each having access to aplurality of lines and each adapted to move step by step in a primarydirection and then step by step in a secondary direction in order toselect a particular oneof said lines, common control equipment connectedto one. of said switches at times over. a set of conductors outgoingfrom said equipment and connected to the other one of said switches atother times over the same set of conductors outgoing from saidequipment, an impulsing circuit extending over certain of saidconductors to the connected switch at each of said times, anothercircuit extending over certain of said conductors to the connectedswitch at each of said times for causing the connected switch to changeover from its primary movement to its secondary movement, meansincluding said circuits where'- by both said movements of one of saidswitches are directed by said equipment and whereby one of saidmovements of the other of said switches is a self-controlled huntinmovement, and a third circuit extending over certain of said conductorsto the connected switch at each of said times for testing the lineselected by the connected switch.

5. In a switching system wherein a connection is extended progressivelyover a train of two motion selective switches; a, control unit forcontrolling each switch in turn to extend the connection to the nextswitch in said trainf said connection including a first circuit outgoingfrom said unit over which impulses are transmitted to each switch inturn tomove that switch step by step, a second circuit outgoing fromsaid unit for operating each switch in turn to change over from itsprimary movement to its secondary movement, and a third circuit outgoingfrom said unit for testing the condition of each switch to which saidconnection is extended by a preceding switch in said train,

6. In a switching system, automatic switches, control equipment commonto said switches, a connection outgoing from said equipment, means insaid equipment for operating each of said switches directively in turnover said connection to extend said connection to another one of saidswitches, whereby said connection is advanced progressively through aplurality of said switches in series, and means in said common controlequipment for testing, over said connection, the condition of eachswitch towhioh said connection is extended by a preceding switch in saidseries.

7. In a switching system, a selector switch, a plurality of groups ofconnector switches accessible to said selector switch, a plurality ofgroups of lines accessible to each of said connector switches, controlequipment, a connection outgoing from said equipment to said selectorswitch, means in said equipment for operating said selector switchdirectively over said connection to select a particular group of saidconnector switches, means for then causing said selector switch tocommence hunting for an idle connector switch in the selected group,testing means in said equipment operated over said connection when anidle connector switch is found for causing said selector switch to haltsaid hunting operation, seize the idle connector switch and extendsaidconnection to the seized connector switch, said first. means thenefiective to operate said connector switch directively first to select aparticular group of lines among the groups accessible to that connectorswitch and then to select aparticular line in the selected group, andmeans in said equipment for testing the selected line over saidconnection todetermine the condition thereof.

8. In a switching system, a selector switch,.a.

plurality or" groups of connector switches accessible to said'selectorswitch, a plurality of groups of lines accessible to each of saidconnector switches, control equipment, two circuits out:- going fromsaid equipment to said selector switch, means in said equipment fortransmitting impulses over one of said circuits to said selector switchthereby to operate said selector switch directively to select aparticular group of said connector switches, means in said selectorswitch then controlled by said equipment over the other of said circuitsfor causing said selector switch to commence hunting for an idleconnector switch in the selected group, means operated when an idleconnector switch is found for halting said hunting operation, seizingthe idle connector switch and causing said selector switch to extendsaid two circuits to the seized connector switch, said: first means thenoperated to transmit impulses oversaid one circuit to operate saidconnector switch di'rectivelyto select a particular group of lines amongthe groups accessible to that connector switch, means in said connectorswitch operated; over said other circuitby said control equipment aftersaid group of lines has been selected thereby'to prepare a line testingcircuit and to condition said connector switch to, select a particularline in the selected group, said first means then operated to transmitimpulses over said one circuit to operate said connector switchdirectively to select a particular linein the selected group, and meanscontrolled over said line testing circuit for determining whether theselected line is idle or busy.

9. In a switching system, a selector switch, a plurality of groups ofconnector switches accessiblev to said selector switch, a plurality ofgroups oilines accessible to each of said connector switches, controlequipment, three circuits outgoing from said control equipment to saidselector switch, means in said equipment for transmitting impulses overthe first of said circuits to operate said selector switch directivelyto select a particular group of connector switches, means in saidselector switch then controlled by said equipment over-the second ofsaid circuits for causing said selector switch to commence hunting foran idle connector switch in the selected group, means in saidequipmentoperated over the third of said circuits when an idle connector switchis foundfor'causing said selector switch to halt its hunting operation,seize the idle (connector switch and extendsaicl' three circuits to theseized connector switch, said first means then operated to transmitimpulses over said first circuit to operate the seized connector switchdirectively to select a particular group of lines among the groupsaccessible to that connector switch, means in said connector switchoperated over said second circuit by said equipment after said group oflines has been selected thereby to condition said connector switch toselect a particular line in the selected group, said first means thenoperated to transmit impulses over said first circuit to operate saidconditioned connector switch directively to select a particular line inthe selected group, and

23 means in said control equipment controlled over said third circuitfor determining whether the selected line is idle or busy.

10. In a switching system wherein a, connection is extendedprogressively through a plurality of switching stages, equipment commonto said stages for controlling the extension of said connection, a twomotion switch at each stage connected in turn to the same set of controlconductors outgoing from said equipment, said set comprising only animpulsing conductor, a guard conductor, a changeover conductor and atest conductor,

11. In a switching system wherein a connection is extended progressivelyover a train of two motion selective switches, a control unit common tosaid switches for controlling each switch in turn to extend theconnection to the next switch in said train, a stepping magnet in one ofsaid switches controlled by said unit to operate said one switch step bystep in one direction, a second magnet controlled by said unit foroperating said one switch in another direction, and means formaintaining said stepping magnet energized during the operation of saidone switch in said other direction.

12. In a switching system wherein a connection is extended progressivelyover a train of two motion selective switches, a control unit common tosaid switches for controlling each switch in turn to extend theconnection to the next switch in said train, 'a stepping magnet in oneof said switches controlled by said unit to operate said one switch stepby step in one direction, a second magnet, a third magnet controlled bysaid unit after the operation of said switch in said one direction forconnecting said second magnet to said unit, said second magnet thencontrolled by said unit for operating said one switch in anotherdirection, and means for maintaining said stepping magnet energizedduring the operation of said one switch in said other direction.

13. In a telephone system wherein a conversa tional circuit is extendedprogressively over a train of automatic switches, a primary steppingmagnet in each switch connected to said conversational circuit andcontrolled thereover to operate that switch in one direction, asecondary stepping magnet in each switch connected to saidconversational circuit and controlled thereover to operate that switchin a different direction, means effective after each of said magnets hasbeen operated for disconnecting that magnet from said conversationalcircuit, and a circuit to which one of said magnets of each switch isconnected when it is disconnected from said conversational circuit, saidone magnet of each switch thereafter operated over said last circuit toinitiate the release of its associated switch.

14. In a switching system, a primary switch having access to a pluralityof secondary switches, means for transmitting impulses alternating inpolarity, impulse receiving means in said primary switch responsive onlyto the impulses of one polarity for operating said primary switchdirectively to connect with a particular one of said secondary switches,and impulse receiving means in the seized secondary switch responsiveonly to the impulses of the other polarity for then operating saidseized secondary switch directively.

15. In a switching system, a pair of numerical switches in serialrelationship to one another, control equipment common to said switches,means in said equipment for transmitting impulses of one character tooperate one of said switches 24 directively and for transmittingimpulses of a different character to operate the other of said switchesdirectively.

said connector thereby to operate said connector switch directively,

1'7. In a switching system, a plurality of switches, control equipmentcommon to said switches, said equipment having outgoing conductors whichare connected in an identical way to different ones of said switches atdifferent times, means in said equipment for impressing voltages of twodiiferent characters alternately upon one of said conductors, impulsereceiving means in a connected oneof said switches responsive only tothe voltage of one of said characters impressed upon said one conductorfor operating that switch directively, changeover means in saidconnected switch operated over another of said conductors after saiddirective operation of said switch, means in the connected switchthereupon connected to said one conductor and responsive only to voltageof the other of said characters impressed thereon for controlling asecondary movement of the connected switch.

18. In a switching system, a plurality of switches, control equipmentcommon to said switches, said equipment having outgoing conductors whichare connected in an identical way to difierent ones of said switches atdifferent times, means in said equipment for impressing voltages of twodifferent characters alternately upon one of said conductors, impulsereceiving means in each of said switches effective when that switch isconnected to said conductors to respond only to the voltage of one ofsaid characters impressed upon said one conductor thereby to impart tothat switch a directive primary movement, other means in each of saidswitches connected to said one conductor after the directive primarymovementof that switch, said other means in one of said switches beingresponsive only to voltage said one character impressed upon saidconductor thereby to impart to said one switch a hunting secondarymovement, and said other means in another of said switches beingresponsive only to voltage of the other of said characters therebytoimpart to said other switch a directive secondary movement.

19. In a switching system, control apparatus having a plurality ofconductors outgoing therefrom, a plurality of switch units eachincluding not more than three electromagnetic devices, means including ajack for detachably connecting each one of said units to said conductorsat times, a plurality of lines accessible to the connected switch unitat each of said times, and means in said apparatus operated forcontrolling the three electromagnetic devices of the connected switchunit to impart to that switch unit a variable primary movement and avariable secondary movement, thereby to select one of the linesaccessible to said. switch unit.

20. In a switching system, control apparatus having a plurality ofconductors outgoing there from, a plurality of switch units eachincluding not more than three electromagnetic devices, means including ajack for detachably connectin -..each one-of said units to saidconductors at times, a plurality of lines accessible to the connectedswitch unit at each of said times, and means in said apparatus operatedfor controlling the three electromagnetic devices of the connectedswitch unit to impart to that switch unit a variable directive movementfollowed by a hunting movement, thereby to select one of the linesaccessible to said switch unit.

21. In a switching system, a plurality of two motion switches eachhaving electromagnets for exerting mechanical forces upon the switchmechanism to operate same, electrical contacts also operatedmechanically by the same electromagnets, circuits individual to eachswitch controlled by said contacts, and a control unit common tosaidswitches for cooperating with the said electromagnets and contacts ofeach switch at times to operate that switch :in two directions withoutthe aid of any other electromagnetic device individual to said switch.

22. In a switching system, a plurality of switches each havingelectromagnets for exerting mechanical forces upon the switch mechanismto operate same, electrical contacts also operated mechanically by thesame electromagnets, circuits individual to each switch controlled bysaid contacts, and a control unit common to said switches forcooperating with the said electromagnets and contacts of each switch attimes to operate that switch successively in three different directionswithout the aid of any other electromagnetic device individual to saidswitch.

23. In a switching system, a plurality of switches each havingelectromagnets for exerting mechanical forces upon the switch mechanismto operate same, electrical contacts also operated mechanically by thesame electromagnets, circuits individual to each switch controlled bysaid contacts, and a control unit common to said switches forcooperating with the said electromagnets and contacts of each switch attimes to operate that switch successively in four different directionswithout the aid of any other electro magnetic device individual to saidswitch.

24. In a switching system wherein a line is extended progressivelythrough a plurality of switching stages, said line comprising only fourconductors, equipment common to said stages, a switch at each stageconnected in turn to said line and controlled thereover by saidequipment to operate successively in at least three directions therebyto extend said line to the next switching stage.

25. In a switching system wherein a line is extended progressivelythrough a plurality of switching stages, said, line comprising only fourconductors, equipment common to said stages, a switch at each stageconnected in turn to said line and controlled thereover by saidequipment to operate successively in four directions thereby to extendsaid line to the next switching stage.

26. In an automatic switching system, a plurality of groups of contactsets, each contact set having two parts which normally are not inelectrical connection with one another, a plurality of outgoing linescorresponding respectively to the different ones of said contact sets,each line being connected to one part of its corresponding contact set,an incoming line connected in multiple to the other part of everycontact set in certain of said groups, another incoming line connectedin multiple to the other part of every contact set in other of saidgroups, contact controlling apparatus, means in said apparatus forreceiving difierent digits at diirerent times and for-selecting acontact set in a diiferentone of said groups at eachof said times, thegroup in which a contact set is selected at each of said times dependingupon the value of the digit received,- and means in saidapparatusoperatedateach oisaid times after a contact set has beenselected for completin an electrical connection between the twosaidparts of said selected contact set.

27. 'In an automatic switching system, a switch in .one exchange havinga plurality 0f groups of contact sets, each contact set having two partswhich normally are :not in electrical connection with one :another, aplurality of trunk lines outgoing :from :said one exchange to connectwith another exchange, said lines -:corresponding .re- "spectively :tothe differenteontact sets in certain of said groups, a plurality oflines local "to said one exchange, said dast llines correspondingrespectively :to the different contact sets in :other of said:groups,.each said outgoing line and each said local line beingconnected to one part of its corresponding contact set, one lineincoming to said switch and connected in multiple to the other part ofevery contact set in said certain groups, another line incoming to saidswitch and connected in multiple to the other part of every contact insaid other groups, contact controlling apparatus, means in saidapparatus for receiving different digits at different times and forselecting a contact set in a different one of said groups at each ofsaid times, the group in which a contact set is selected at each of saidtimes depending upon the value of the digit received, and means in saidapparatus operated at each of said times after a contact set has beenselected for completing an electrical connection between the two saidparts of said selected contact set.

28. In an automatic switching system, a plurality of groups of contactsets, each contact set having two parts which normally are not inelectrical connection with one another, a plurality of out-going linescorresponding respectively to the different ones of said contact sets,each line being connected to one part of its corresponding contact set,a repeater, an incoming line connected directly in multiple to the otherpart of every contact set in certain of said groups and connected viasaid repeater in multiple to the other part of every contact set inother of said groups, contact controlling apparatus, means in saidapparatus for receiving diiferent digits at different times and forselecting a contact set in a different one of said groups at each ofsaid times, the group in which a contact set is selected at each of saidtimes depending upon the value of the digit received, and means in saidapparatus operated at each of said times after a contact set has beenselected for completing an electrical connection between the two saidparts of said selected contact set.

29. In a telephone system, an exchange, a selective switch in saidexchange having access to local lirTes and to outgoing lines, controlequipment, a conversational circuit incoming ,to said 27' in seriestothe seized local line, and means controlled at-said other times todisconnect said incoming'conversational circuit from said controlequipment and connect it through said switch independently of saidcontrol equipment to the seized outgoing line.

30. In a switching system, an exchange, a selective switch in saidexchange having access to local lines and to outgoing lines, a repeater,a circuit incoming to said repeater over which impulses indicativeoi alocal call are received at times and over which impulses indicative ofan outgoing call are received at other times, means in said repeater forrepeating part of the receivedimpulses to said switch, whereby. saidswitch is effective to seize a local line at said first times and toseize an outgoing line at other times,.said means in said repeater thenoperated at said first times to repeat the balance of said receivedimpulses over said switch and the seized local line, means controlled atsaid other times after said switch has been operated to seize anoutgoing line for disconnecting said incomin circuit from said repeaterand connecting it through said switch directly to the seized outgoingline, whereby the balance of said received impulses are conducted fromsaid incoming circuit directly over said switch and said outgoing linewithout repetition.

GEORGE THOMAS BAKER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

